Metals and Magnets

Metals and magnets used to make many common modern machines often fulfill very

rigid specifications. These specifications often relate to their ability to be molded into

desired shape; electrical conductivity or magnetic susceptibility. Thanks to the research

efforts of millions of scientists during the past century. Today very often we have a

choice in selecting such materials. Metals and magnets abound everywhere around us.

They are the essential constituents of modern civilization. No wonder, every science

student is required know about them, but still as usual several doubts about them do

persist. For example:

Metals

• Mercury is a metal but why is it found in liquid state?.

• Even though it is a liquid, why does mercury not wet the wood, floor or iron?

• Why can't we make precious elements like gold, platinum, uranium etc. by

placing electrons, neutrons protons into other elements?

• Do all the metals exhibit the photoelectric effect?

• Why are the TV antennas made of aluminum and not of any other metal or a

non-metal?

• Why do the copper and bronze utensils turn green or blue if they are kept out

for quite some time?

• A piece of wood catches fire but a piece of iron doesn't. Why?

• When electricity is passed through copper wire, does the copper atoms get

ionized to Cu++ ions?

• When we heat a metal why does it become red?

• Why does an iron nail get a copper coating on its surface, if kept for some

time in copper sulfate solution?

• Why do copper; aluminum or silver do not become superconductors, though

they are good conductor of heat electricity, but poor metals like Tantalum lead

are very good superconductors?

• When we temper soft iron why does it become hard?

• Why do we use only copper wire in a dynamo to produce electric current, why

not any other material?

Metals are chemical elements which share a set of physical and chemical properties.

For example, all metals reflect light and have shiny appearance. They also are good

conductors of electricity and heat. Most metals are malleable -- that is, they can be

hammered into thin sheets. Most metals are also ductile, which means that they can be

drawn into wires. But being a solid is definitely not a prerequisite for an element to be

classified as a metal. One should therefore not be surprised to discover that liquid

mercury is a metal too. In fact mercury is not the only metal which is in the liquid state

at room temperature; gallium and cesium also have melting points around 30 degrees

Celsius. In order to wet the surface of another substance, all that is required of a liquid

is that its atoms (or molecules) must be strongly attracted towards the atoms

(molecules) of the other substance. The atoms of mercury do not have strong

interaction with the cellulose molecules in a piece of wood or the atoms of iron hence

it does not wet their surface.

Although it is true that in principle the atoms of any element can be

transformed into atoms of another element by nuclear transformations, in practice it is

often so expensive that it is not even worth trying. Often the energy required to carry

out the reaction is too high, or the nuclear reaction invariably results in a mixture of

isotopes of several elements, which besides being radioactive, cannot be separated by

normal chemical processes. It is therefore not economical to prepare gold, silver or

platinum by bombarding other elements like copper or aluminum with other atoms or

subatomic particles.

The removal of electrons from a metal by irradiation with light is known

as the photoelectric effect. This requires that the energy quanta delivered by the light

used for irradiation exceeds the energy which holds some electrons inside the lattice of

atoms of the metal. The range of energy quanta of visible light is rather limited and is

not be sufficient to remove electrons from all metals. It is for this reason only a few

metals, e.g., germanium and cesium show photoelectric effect.

The choice of a particular metal for making a particular product is often

dictated by several considerations such as ease of fabrication, durability and cost.. The

cost of a particular metal depends on the relative abundance of its ores and the cost of

extracting the metal from the ore. Thus aluminum, which nowadays is among the

cheaper metals, used to be considered a precious metal during the past century despite

the fact that its ores are amongst the most abundant. Only after economical methods of

its refining were discovered, that it became so cheap.

The primary requirement for making a TV antenna is a metal tube which can

be easily bent. Aluminum is preferred for making TV antennas, because it is cheaper

(than most other elements except iron) and also corrosion resistant (while iron or steel

tubes are not). In contrast, copper wires are often used for coils in electric motors.

This is so because copper is much more ductile than aluminum and the electrical

conductivity of copper is also higher than that of aluminum. More compact and energy

efficient electric motors or dynamos can therefore be designed using copper coils.

Aluminum wires are nevertheless used nowadays in several brands of ceiling fans or

the ballast for tube-light.

The conduction of electricity through a metal is normally explained in terms of

a model for the binding of atoms in its lattice. According to this model, the atoms of a

metal are bound together by the so called metallic bond, whereby the electrons from

each atom are pooled together and fill up energy levels for the entire lattice. The

electrons in the highest occupied energy levels are responsible for the conduction of

electric current through a metal. If these levels are not totally filled up, electrons in

these levels can be excited easily and they can move around the lattice freely. The free

movement of the electrons is responsible for the conduction of electricity. Since

conduction of electricity involves only movement of electrons through the wire, there

is no question of loss of electrons from atoms or the formation of ions e.g. Cu++ in

case of copper.

When a piece of solid metal is heated, its atoms start vibrating more violently.

If the metal has a sufficiently high melting point so that it does not melt, above a

certain temperature the energy of these vibrations is sufficient to excite the electrons in

the highest occupied energy levels to higher levels. These excited electrons can often

emit their excess energy in the form of light radiation. The colour of the light radiated

depends on the energy radiated. Thus we can explain why a piece of iron when heated

to a high temperature looks red.

It is not true that iron does not burn, we often see rail wagons or iron

components of a house on fire. A substance is said to burn when it reacts with another

element (like oxygen or chlorine) with the simultaneous evolution of heat (which

sustain the process of burning). Wood burns at a relatively low temperature, but iron

can also similarly react with oxygen to form oxides albeit at a much higher

temperature. However, there are differences, burning of wood, as we all know, is

accompanied by the production of some gases which at high temperatures emit light --

a flame. It is also self sustaing as the heat evolved by the combustion of a small

portion is sufficient to promote the combustion of the rest. Burning of iron, on the

other hand, does not generate a flame, because no gas is evolved in the reaction. Also

the combustion of iron may not self sustaining in a small fire.

When an iron nail is dipped in a copper sulfate solution, the copper ions in the

solution have a choice. They can either remain as ions in the solution or they can

extract a few electrons from the iron nail and in the process get transformed into

neutral atoms deposited on the surface of the nail. It so happens that Nature favors the

latter process. Hence, it is the copper from the solution which gets deposited on the

iron nail. Similar electrochemical process is possible for many other pairs of atoms.

When copper is exposed to air, which contains oxygen, carbon dioxide, and water, it

can react with them to form basic copper carbonate, which is green in colour similarly

in the presence of some sulfur dioxide some basic copper sulfate (bluish green) is

formed.

Normally, all metals conduct more electricity when the temperature is lowered.

This is so because at lower temperatures the less violent vibrations of the atoms in the

lattice offer lesser resistance to the flow of electrons. Thus increase in electrical

conductivity of metals can be predicted from the popularly accepted model of metals.

However, the dramatic increase of conductivity at very low temperature (about –

2700C), known as the superconductivity could not be easily explained by these models.

It is not true that only tantalum or lead exhibit superconductivity, aluminum or silver

also become superconducting at very low temperatures.

Magnets

Why do pieces of iron get attracted towards a magnet?

How is a magnet made? What are its constituents which make it attract iron?

Why do magnets lose their magnetism when heated or it struck with a hammer?

When we rub an iron nail on a bar magnet in haphazard way, it does not get

magnetized as powerfully as when it is rubbed in a particular direction on the bar

magnet. Why is it so?

If we take a magnet and cut it into half the halves behave as individual magnets

and attract each other. Why does this happen?

Why does a magnet attract objects made up only of iron, nickel or cobalt and not

any other metal?

Do magnets conduct electricity? If no, why? If yes, what happens to the

magnetic lines of forces?

Is it possible to make materials which attract metals like copper, aluminum and

others which are not attracted by iron?

Can there be a spherical magnet? If yes, how are its poles located?

Before we attempt to answer such queries we must know what is magnetism and what

is its genesis.

A magnet is a substance, not necessarily an element or a metal, that has a characteristic

property of attracting or repelling certain objects. Quite often these objects are made

up of iron. The force with which a magnet attracts another object is often more than

the gravitational force between them, it also does not depend on the presence of an

electric charge.

The genesis of magnetic force in a substance can either be traced to an electric

current flowing through/around it or to a basic attribute of fundamental particles that

constitute all matter. Just as the gravitational force responsible for the motion of a ball

down a hill slope is due to its mass and the electrostatic attraction of pieces of paper

towards a plastic comb (which has been rubbed on a woolen cloth) is due to the

electric charge, the magnetic force responsible for the motion of an iron piece towards

a magnet is due to another attribute of all fundamental particles namely spin. However,

although it often helps to understand some characteristics of this attribute by drawing

an analogy to a spinning object, strictly speaking, spin of an electron or proton cannot

be associated with any kind of their motion. They do not spin around any axis like

Earth; the Moon or a cricket ball do. This is so because as we already know, according

to the currently accepted theory of matter, electrons etc. are not necessarily particulate.

While we can explain some of the observations about them assuming them to be

particles, there are many other observations which are better explained if we assume

them to be waves. When we talk of a spinning electron or a proton we really do not

know whether it is spinning eastward or northward, whether the angular velocity of

this spin is x or y.. All we can say , is that this spin can have only two values +1/2 and

–1/2, the positive and negative signs are used to differentiate between the two

directions, e.g., upwards and downwards.

The magnetic property of a substance is attributed to the net spin of electrons

of its atoms. In the various energy levels in an atom, the spins of some electrons pair

up while that of some others may remain unpaired. Elements whose atoms have some

electrons with unpaired spin are known as paramagnetic, while others in which they are

all paired up are known as diamagnetic. Paramagnetic substances are attracted weakly

by magnets, while diamagnetic substances are weakly repelled by magnets. While

aluminum, chromium sodium and oxygen are examples of paramagnetic substances;

copper; lead; mercury and nitrogen are examples of diamagnetic substances. Elements

popularly associated with magnets, namely iron; cobalt or nickel are classified in a

separate category altogether: the ferromagnetic substances. An interaction which can

best be explained in terms of quantum mechanics, leads to the coupling of spins of

adjacent atoms in a ferromagnetic solid. This results in the creation of the so called

‘local magnetic domains’ in which spins of all atoms are parallel, that is, magnetization

is unidirectional. A typical domain contains 10 17 to 1021 atoms and occupies a

volume on the order of 10-12 to 10-8 m3. Adjacent domains are magnetized in

different directions, so that there may be no net bulk magnetization. Thus the volume

of a piece of iron can be subdivided into regions (magnetic domains) which are like

microscopic magnets pointing in a particular direction. If the effect of each of these

domains cancel each other the iron piece is not a magnet, that is, it will not attract

other pieces of iron towards it strongly. However, if the magnetic effect of these

domains do not cancel each other the iron piece will be a magnet.

We can now safely attempt to answer the above queries. The attraction or

repulsion of bodies due to magnetism is analogous to attraction/repulsion due to

electric charges. Just as there can be two kinds of electric charge on a body and

oppositely charged bodies attract and similarly charged bodies repel, a magnet also has

two poles, normally known as north and south poles. However, while negatively and

positively charged particles can exist independently, the two poles of a magnet cannot.

A magnet always exists as a dipole. If the north pole of one magnet comes closer to the

south pole of another the two magnets are attracted towards each other and will tend

to come closer. In case the poles closer to each other are the same, the magnets tend to

move away from each other. When a permanent magnet is brought close to an iron

nail, it is attracted towards it because the magnet induces the alignment of some of the

magnetic domains in the nail, thus converting it into a magnet too, albeit temporarily.

To convert a piece of metal into a magnet, does not require addition of any

substance to it. If a strong permanent magnet is moved across a piece of hard magnetic

substances like iron, cobalt or nickel in any particular direction, the metal piece will

acquire magnetic properties. This is so because in this process either some magnetic

domains get aligned or some domains aligned in the direction of movement of magnet

grow larger. This conversion can also be effected by hammering or tapping in close

vicinity of a magnet. Often iron pieces become weakly magnetic on hammering

because the Earth is also like a huge magnet. Hammering a strong magnet, on the other

hand, results in the loss of some of its magnetic property, because in this case some

domains lose alignment. Similar is the case when a magnet is heated.

If a magnet is broken into two pieces, each piece becomes a magnet, because

each will have similarly aligned magnetic domains.

It is not true that a magnet will attract only objects made up of iron, cobalt, or

nickel. The number of substances that exhibit magnetic properties far exceeds three. In

fact the naturally occurring magnets,; the lodestones, are not pure iron, rather magnetite (Fe3O4). Also there are several alloys and oxides of metal which exhibit

strong magnetic properties. For example, ferrite’s ( e.g. BaO:6Fe2O3); Alinco alloy

(cobalt-platinum or manganese-Bismuth) are all magnetic. In fact most of the magnets

used for popular electrical products are not made of pure iron; nickel or cobalt but

various ceramic materials.

Magnets, made up of a pure metal, do conduct electricity. Magnets, made up of

oxides of metals, on the other hand are often poor conductors of electricity. As already

indicated aluminum is a paramagnetic substance, hence it is attracted weakly toward a

magnet. Copper on the other hand is diamagnetic and hence is not usually attracted

towards magnets. However, as we all know the motion of an electric motor is due to

the motion of copper coils in it. This is because when an electric current is passing

through a copper or aluminum wire coil, it behaves like a magnet and will be attracted

towards a magnet.

One can indeed make a spherical magnet. The poles of such a magnet are located

at any two diametrically opposite points.